1. Field of the Invention
The present invention relates to the selective hydrogenation of benzene in a stream, such as naphtha to make cyclohexane and reduce the benzene content. More particularly the invention relates to a process wherein the hydrogenation of the benzene and separation of the cyclohexane product by distillation is carried out simultaneously in a distillation column reactor. More particularly the invention relates to a process wherein the benzene is contained in a light naphtha stream from a cracking process or from a catalytic reformer wherein the stream also contains toluene. Most particularly the invention relates to a process for the hydrogenation of the benzene contained in a light naphtha stream with very little hydrogenation of the toluene or other aromatics.
2. Related Information
The reduction in the lead content of gasolines and the ban on use of lead anti-knock compounds have lead to a search for other ways to improve the octane number of blending components for gasoline. The alternatives to uses of lead anti-knock compounds are chemical processing and the use of other additives.
One common process long used by the refinery industry to upgrade raw naphtha to high octane gasoline is catalytic reforming. In catalytic reforming the raw naphtha having a boiling range of circa 115.degree.-350.degree. F. is passed over an alumina supported noble metal catalyst at elevated temperatures (circa 920.degree.-950.degree. F.) and moderate pressure (circa 200-550 psig). The catalyst "reforms" the molecular structures of the hydrocarbons contained in the raw naphtha by removing hydrogen and rearranging the structure of the molecules so as to improve the octane number of the naphtha. However, the increase in octane number also reduces the liquid volume of the naphtha as the specific gravity is increased.
Because of the multiplicity of the compounds in the raw naphtha, the actual reactions which occur in catalytic reforming are numerous. However, some of the many resulting products are aryl or aromatic compounds, all of which exhibit high octane numbers. The aryl compounds produced depend upon the starting materials which in a refinery are controlled by the boiling range of the naphtha used and the crude oil source. The "reformed" product from a catalytic reforming process is commonly called reformate and is often separated into two fractions by conventional distillations--a light reformate having a boiling range of circa 115.degree.-250.degree. F. and a heavy reformate having a boiling range of circa 250.degree.-350.degree. F. The aryl compounds in each fraction are thus dependent upon their boiling points. The lower boiling or lighter aryl compounds, e.g., benzene, toluene and xylenes, are contained in the light reformate and higher boiling aryl compounds are contained in the heavy reformate.
The light reformate is that portion formulated into gasoline. Until the EPA mandate requiring an elimination of most benzene from gasoline (general requirements for reformulated gasoline specify a maximum of 1.0 vol. % benzene) this was a solution to the elimination of lead. Now benzene must be removed or converted to more benign components, while maintaining the octane of the gasoline. One effective means to achieve this is to alkylate the benzene, however the olefin streams for this purpose may be expensive or otherwise employed.
Hydrogenation of the benzene is an alternative for removing that aromatic compound from gasoline streams. One example of this process is disclosed by Hsieh, et al in U.S. Pat. No. 5,210,348 wherein hydrogenation of the benzene fraction is used alone or in combination with alkylation.
Peterson in U.S. Pat. No. 2,373,501 discloses a liquid phase process for the hydrogenation of benzene to cyclohexane wherein a temperature differential is maintained between the top of the catalyst bed where benzene is fed and the outlet where substantially pure cyclohexane is withdrawn. The temperature differential is due to the change in the exothermic heat of reaction released as less and less benzene is converted as the concentration of benzene decreases. Hydrogen is supplied counter current to the benzene/cyclohexane flow. Temperature control coils are disposed within the reactor to maintain the temperature differential if the exothermic heat of reaction is not sufficient or to cool the bed if too much heat is released. Peterson recognizes that although the bulk of his reaction takes place in the liquid phase a portion of the benzene and cyclohexane will be vaporized, especially near the top of the reactor where the benzene concentration is highest and conversion is highest. A reflux condenser is provided to condense the condensible material and return it to the reactor. Thus a substantial portion of the heat of reaction is removed by condensation of the reactants vaporized throughout the reaction.
Larkin, et al. in U.S. Pat. No. 5,189,233 disclose another liquid phase process for the hydrogenation of benzene to cyclohexane. However, Larkin, et al utilize high pressure (2500 psig) to maintain the reactants in the liquid state. In addition Larkin, et al disclose the use of progressively more active catalyst as the concentration of benzene decreases to control the temperature and unwanted side reactions.
Hui, et al. in U.S. Pat. No. 4,731,496 disclose a gas phase process for the hydrogenation of benzene to cyclohexane over a specific catalyst. The catalyst reported therein is nickel supported on a mixture of titanium dioxide and zirconium dioxide.
The plug flow or single pass reactor for the hydrogenation of benzene in a naphtha stream has as a drawback the problem of indiscriminate hydrogenation of all the aromatic compounds which can contribute to the improved octane.